1. Field of the Invention
The present invention relates to a double-sided connector that, when an IC package such as a CPU or an LSI, an LGA package, a BGA package or the like is mounted on a printed circuit board, can absorb variations in the height of contact terminals, without generating warping in the package and the printed circuit board.
Priority is claimed on Japanese Patent Application No. 2008-171004, filed Jun. 30, 2008, the contents of which are incorporated herein by reference.
2. Description of Related Art
Conventionally, technology for mounting an integrated circuit (IC) package such as a central processing unit (CPU) or large scale integrated circuit (LSI) on a printed circuit board via a socket has been considered. For example, in most personal computers and servers, sockets are used to mount land grid array (LGA) or a ball grid array (BGA) package type of CPUs on their motherboards.
Recent years, in accordance with the improvements in the function and the performance of the CPUs, the number of pins, the working speed, and the package size has been increasing, and the pitches between the pins has been getting finer.
These developments of CPUs require the necessity of countermeasures for: a socket to accommodate the increasing number of pins; absorbing greater warping of the package generated by increasing size of the package; and absorbing variations in the heights (i.e., the coplanarity) of contact lands and solder balls of the package. Therefore, it is necessary to secure enough movable range of the pins of the socket contact.
In order to realize the finer pitches of the packages, size reduction in the socket contacts is important, and also it is necessary to assure contact with appropriate contact pressure between the pins of ICs and the socket contacts.
Furthermore, in order to respond to the higher working speed of the CPUs, it is important for the socket contacts to have a lower inductance. The socket contacts are also required to have a lower contact resistance and a larger permissible current in response to increasing current consumption caused by the higher working speed.
Most sockets for LGA packages presently used include 400 to 800 pins at a pitch of approximately 1 mm, and are manufactured by complexly folding a metal plate to form a socket contact having a predetermined shape and then by inserting the socket contact into a socket housing (see Japanese Unexamined Patent Application, First Publication No. 2004-158430 and Japanese Unexamined Patent Application, First Publication No. 2005-19284).
In the sockets having the above-mentioned structure, each of the metal socket contacts also serves as a plate spring which generates an appropriate load at a predetermined stroke to obtain a stable contact resistance. In addition, in this structure, while increasing the load applied on the socket contacts to obtain the predetermined contact pressure, a wiping effect can be obtained in which a contact point between the socket contact and the contact land of the IC moves, and thereby removing foreign bodies on surfaces of the socket contact and the contact land of the IC.
However, since this type of socket contact for an LGA package basically has a cantilever spring structure, it was difficult to realize finer pitches.
This is because the length of a cantilever spring portion of the socket contact needs to be shorter in order to realize the finer pitches. However, if the length of the cantilever spring portion having the same material and the same shape is shortened in relation to the original length, the load to be applied thereon for obtaining a predetermined stroke needs to be increased. If the wire diameter of the cantilever spring portion is reduced to an appropriate value to lower the load to be applied thereon, problems arises such as: the cantilever spring portion will deform in excess of an elastic deformation range thereof, and then will have a plastic deformation, when the predetermined load is applied; and the required allowable stress decreases and this results in incapable of withstanding the predetermined load. The reason for this is that, while in a range where the allowable stress is proportional to the wire diameter of the cantilever spring portion, the spring constant which determines the allowable load (or the load corresponding to the predetermined stroke) is proportional to the cube of the wire diameter of the cantilever spring portion.
Accordingly, instead of the above-mentioned structure which obtains the predetermined contact pressure using the cantilever spring portion, another structure is proposed in which: a metal contact portion is designed so as be capable of performing the plastic deformation; and applying a repulsive force to the metal contact portion by utilizing a rubber or an elastomer.
For example, a structure is proposed that uses a flexible printed circuit board to realize functions of the socket contact in which: an elastomer is sandwiched between two flexible printed circuit boards; and upper and lower layers of the flexible printed circuit boards are electrically connected by soldering metal pins (see Japanese Unexamined Patent Application, First Publication No. 2004-71347).
Also, another structure is proposed in which: a metal film is plated on an elastomer having predetermined dome-shaped portions and through-holes which were molded in a mold in advance; and a circuit that electrically connects the through-holes and contact points on the dome-shaped portions is formed by photolithography (see Japanese Unexamined Patent Application, First Publication No. 2001-332321).
Recent advancements in micro-mold technologies enable designing molds capable of molding at micron-order precision. If this type of mold is applied to the above-mentioned structure that plastically deforms the metal socket contact, and supplements the repulsive force with the rubber or the elastomer, it would be possible to collectively fabricate all or a plurality of the contact terminals each having a shape of a terminal on the elastomer, by batch process.
Therefore, all or a plurality of pins can be collectively formed by batch process, by photolithography and etching in a conductive circuit portion and a metal contact part of each terminal, after forming of conductive portions by plating and electrically connecting between layers by plating the through-holes. Therefore, it is in line with the market demand for a greater numbers of pins.
Furthermore, applying circuit-forming technologies by photolithography and etching are also suitable for obtaining finer pitches.
In the above-mentioned conventional structure where the elastomer is sandwiched between the two flexible circuit boards requires the two flexible circuit boards, the metal pins for electrically connecting between the layers, and soldering via metal pins between the two flexible circuit boards, and the like. Therefore, the manufacturing processes are consequently complex, and the manufacturing cost thereof is high.
In addition, in the above-mentioned structure including the elastomer formed with the dome-shaped portions and the through-holes, if this structure is pressed along the thickness direction thereof, the dome-shaped portions will be deformed in accordance with the load applied thereon and may be collapsed. In such case, the wiping effect cannot be expected although it is required as one of the functions of the socket contact. In addition, increasing the number of pins necessitates absorbing: warping of the printed circuit board and the IC package; and variations in the height of the contact lands. However, in the above-mentioned structure, the amount of motion for absorbing the warping and variations in the coplanarity is limited by the height of the dome-shaped portion.
Furthermore, in the above-mentioned structure in which: the metal film is plated on the elastomer; and the circuit that electrically connects between the through-holes and contact points on the dome-shaped portions is formed by photolithography, stress may be generated and concentrated in a part of the metal plating portion on the elastomer.
Furthermore, in the above-mentioned structure where the metal film is plated on the elastomer, since the movable range of the point of contact part is limited by the range of deformation of the elastomer along its thickness direction, enlargement of the movable range cannot be expected.
The present invention was made in view of the above-mentioned circumstances, and has an object of providing a double-sided connector which can absorb variations in the height of point of the contact part caused by warping of the package and the printed circuit board and the variations in the height of the contact terminals, which can reduce concentration of stress in the through-holes which electrically connect between the layers, which can enlarge the movable range of the point of contact part, and which can thereby meet the recent demand for finer pitches.